1. Field of the Invention
The present invention relates to a process for radio frequency plasma etching, and more specifically to a process for etching a silicon substrate with plasma generated by a radio frequency discharge to form a recess or opening on a surface of the substrate in the fabrication of a micro-processed silicon device.
2. Description of the Related Art
When fabricating micro-processed silicon devices, such as pressure sensors, a technology for etching a substrate vertically and deeply is essential in order to form a recess used for detecting a variation in pressure or for element isolation or to form an opening for making a electrical contact between conductive layers.
Conventionally known technologies for forming a recess or opening include a wet etching process using a reactive solution such as potassium hydroxide and a plasma etching process, particularly a reactive ion etching process using a plasma containing ions of a reaction gas.
In wet etching, it is difficult to control the angle of the side wall of a recess or vertical opening. This is due to the fact that the etching rate varies depending on the plane orientation of the silicon. As the diameter of the silicon wafer, the thickness of the silicon wafer, and thus the amount of the silicon wafer to be etched increases, it becomes more difficult to the shape of an etched portion. For example, the etched portion has a tapered shape along the crystallographic plane due to the reasons discussed above. As a result, the size of a sensor, etc. has to be made larger more than necessary.
In contrast, in plasma etching, the etching rate does not depend on the plane orientation of a. In addition, plasma etching has an etching rate that is equal to or higher than that of the wet etching. The etching rate for a single silicon by dry etching is around 2 .mu.m/min. Nevertheless, the plasma etching does not permit batch treatment of a large number of wafers and thus the treatment efficiency is inferior.
In dry etching, the etching rate may be increased by increasing the supplied power, the pressure of a reaction gas or the flow rate of a reaction gas or by heating the substrate, but these operations simultaneously increase the side etching, to make the side wall of the etched portion more tapered, and a vertical side wall can not be obtained. Moreover, an increase in the ion energy simultaneously increases damage to the etching surface of the substrate and the problem of surface roughness arises. Moreover, in the case of deep etching, a mask material, such as aluminum, having a high etching selectivity to silicon becomes necessary due to the large amount of. However, the mask material, when etched at a high ion energy, may be sputtered and scattered again (re-deposited) onto the substrate to increase the surface roughness. Accordingly, in practice, it is necessary to adopt conditions in dry etching to obtain a vertical shape and a less roughened surface, and an enormously long time is required in fabricating a sensor or the like involving deep etching.